OR/17/062 Focal mechanism
| Baptie, B, Ford, G, and Galloway, D. 2017. The Moidart earthquakes of 4 August 2017. British Geological Survey Internal Report, OR/17/062. |
Local mechanisms for the earthquake were determined from measured first motion polarities on the vertical component of ground motion and SH/P amplitude ratios using three different methods: FOCMEC, the grid search method of Snoke et al. (1984)[1]; HASH (Hardebeck and Shearer, 2003[2]); and, FPFIT, Reasenberg and Oppenheimer [1985][3]. Both FOCMEC and HASH use both polarities and SH/P amplitude ratios. FPFIT uses only polarities. The 1-D velocity model and the best fitting hypocentre were used to determine take-off angles.
We measured 11 first motion polarities and 5 SH/P amplitude ratios. The stations where these were measured are shown in Figure 8(a) and (b). Sampling of the focal sphere is quite poor as a result of the large azimuthal gap with all the measurements from either the east of the epicentre.
For the grid search method of Snoke et al. (1984)[1], we used a grid spacing of 2° and a maximum amplitude ratio error of 0.1. 21 solutions were found that fitted the data. These were all very similar and we selected the one with the smallest amplitude ratio error as representative. We also used a grid spacing of 2° for HASH along with a maximum average error in the amplitude ratio of 0.1.
The source mechanisms given by the three different methods are shown in Figure 8(c), (d) and (e). All three methods show a right-lateral predominantly strike-slip fault that strikes north-northwest south-southeast and dips by varying amounts to the west. The second fault plane calculated by both FOCMEC and FPFIT strikes approximately east-west and dips north. However, HASH calculates a second fault plane that is near vertical and strikes northeast-southwest (d). These results suggest that the mechanism is not very well constrained.
References
- ↑ 1.0 1.1 SNOKE, J, MUNSEY, J, TEAGUE, A, and BOLLINGER, G. 1984. A program for focal mechanism determination by combined use of polarity and P-SV amplitude ratio data. Earthquake Notes 55, 3–15.
- ↑ HARDEBECK, J L, and SHEARER, P M. 2003. Using S/P Amplitude Ratios to Constrain the Focal Mechanisms of Small Earthquakes. Bulletin of the Seismological Society of America, 93:2434–2444.
- ↑ REASENBERG, P, and OPPENHEIMER, D. 1985. FPFIT, FPPLOT, and FPPAGE: Fortran computer programs for calculating and displaying earthquake fault plane solutions. Technical report, U.S. Geol. Survey.